Mooring of offshore objects typically requires multiple mooring lines between the platform and the sea floor, and suitable anchors on the sea floor connected to the mooring lines. Mooring systems currently used include catenary mooring illustrated in FIG. 1 and taut mooring illustrated in FIG. 2. FIG. 3 illustrates a known mooring line spread.
In catenary mooring, catenary mooring lines resist horizontal forces by the weight of the lines, which are usually steel chain or wire rope. As the top of the line moves horizontally, the sag in the catenary is reduced and the horizontal restoring force increases. The horizontal stiffness of the line is directly related to the weight of the line relative to the horizontal force.
In taut mooring, taut mooring lines resist horizontal force by stretching. The lines are relatively light (for example synthetic rope), and remain relatively straight under tension. The horizontal stiffness is directly related to the stiffness of the line, not to its weight.
Anchors for these types of moorings must be designed to take the maximum horizontal and vertical loads at the seabed exerted by the lines. Catenary moorings generally are designed so that the anchors do not experience any vertical loads (“uplift”). This requires longer line length (“scope”) than a taut mooring, and requires a larger area on the seafloor assigned for the mooring spread, i.e. a larger “foot print”. Mooring of offshore objects in deep water in recent years has favored taut moorings because the reduced cost for mooring lines and the smaller foot print. However, taut mooring requires anchors that are able to withstand uplift.
There are many common types of anchors in use today. The drag embedment anchor is commonly used with catenary moorings for temporary systems such as drilling rigs. Most permanent moorings in use today use a taut mooring and a piled anchor, either driven with underwater pile hammers, or installed using a suction principle. These “suction anchors” are installed by setting them vertically on the seabed to create a seal around their base. By pumping water from their interior a large pressure differential is achieved which results in the pile being “sucked” into the seabed.
Many conventional anchors require a thick layer of suitable sediment or sand to function. They are not suitable for rock bottoms, or for areas with highly organic sediment, e.g. coralific or calcareous sediments. For these types of areas, gravity anchors or bored piled anchors are available. Two forms of a gravity anchor include a grillage overlaid with rock or iron ore, or a gravity box filled with ballast. Gravity anchors resist uplift with weight, and horizontal forces with bottom friction.
Gravity anchors need to be very heavy to resist vertical and horizontal loads. If the anchors are deployed in deep water, the only vessels capable of lifting and lowering these heavy anchors may be large derrick vessels costing several hundred thousand dollars per day. Alternately, a lighter box or grillage may be lowered with a smaller cheaper vessel, but ballast must then be installed.
Bored piled anchors typically require an expensive drilling vessel for installation. In addition, bored piled anchors require mooring line attachment above the seabed. It is not generally favorable to attach the mooring line at the top of the pile because of the large bending moment that is imparted to the pile from the eccentricity of the horizontal force. The attachment point is typically below the sea floor, close to the center of the reaction force from the soil. This presents a difficult design issue for piles that are bored into rock, as it becomes impossible to attach the mooring line below the seabed. For this reason, drilled anchors are more commonly used as tension piles.